The casting of a pourable material into an open mold is perhaps the oldest fabrication technique still being used by the plastics industry. Encapsulation, potting and casting are all similar processes used to produce different types of products.
Encapsulation or embedment, is generally understood to refer to embedding an object completely in a plastic material. A rose or commemorative coin that is encapsulated in an acrylic resin is an example. A cardiac pacemaker is encapsulated in a silicone resin in order to render the parts compatible with live tissue. Encapsulation is normally achieved with a reusable mold.
Potting is very similar, except that the mold normally becomes a part of the final product. Placing delicate electrical components inside a plastic box and pouring plastic into the space between the box and the components is a type of potting.
Encapsulation and potting have the same end objective of protecting the enclosed components from external, mechanical or environmental attack. Crystal clear acrylic is used to display, capture and hold the otherwise temporary beauty of a flower. Electrical components which must be visually inspected or read through the potting material are potted in clear acrylic or transparent polyester or epoxy. If protection against impact or shock loading is important, the parts may be surrounded by a self-rising urethane foam. Silicones are used to provide an excellent moisture and heat resistance.
Epoxy resins, because of their dielectric strength, low shrinkage upon cure, good adhesion, and ability to retain properties under varying environmental conditions are reasons why the electrical and electronic industry is a major consumer of such epoxy resins.
Epoxy resins also exhibit good volume resistivity, low dissipation factor, and good dielectric strength as well as high mechanical strength at elevated temperatures and high humidity. Semi-conductors, transistors, and integrated circuits use highly filled epoxies because of the high temperature resistance, low shrinkage, and reliability needed in these electronic devices. (See Modern Plastics Encyclopedia, 1982, 1983, Page 34, Modern Plastics Encyclopedia, 1983-1984, Page 24.).
For most uses, epoxy resins are filled with a particulate inorganic filler, especially silica. The silica may be fumed silica, hydrated silica, silica aerogels and silica xerogels, and have an average particle size in the range of 0.05 to 20 microns. In addition to decreasing costs, these fillers serve to increase hardness, act as a heat sink for the exothermic curing reaction, decrease shrinkage during curing, add opacity and color, and improve other general processing and performance parameters. Silanes are extensively used in many electrical grade silica-filled epoxy resin systems (Moreland, Netherlands Pat. No.: 6901148 filed 23 Jan. 1969). The primary purpose is retention of electrical insulation and low loss properties after extensive exposure to water. The use of 1% epoxysilane-treated, naturally occurring silica in an electrical epoxy resin system has been shown to provide improvements in both and dry flexural strength and complete retention of electrical properties after extensive water immersion (Moreland, Ibid.). (See the article in "Composite materials" by Broutman and Krock, Volume 6, the article entitled "Silane Coupling Agents in Particulate Mineral Filled Composites" at Page 143 ).
Reference may also be had to U.S. Pat. No. 4,151,154 which discloses metal oxide particles, e.g., silica, treated with a polyether substituted silicon compound alone or in combination with a different silane having the formula R.sub.n.sup.3 --Si(X.sub.4-n).sub.b where R.sup.3 is an organic radical of 1 to 18 carbons and X is a hydrolyzable group, e.g., alkoxy. These silicas so treated are used in various resins including epoxies. U.S. Pat. No. 4,414,272 discloses various polyester resins containing silica particles treated with a silane.
It has now been found most surprisingly that several terpene derived silane esters are decidedly superior to materials currently used to provide good electrical properties in silica/epoxy composites when exposed to hot water. This is a test usually carried out to predict long term properties in a humid environment.
These silane esters can be added directly to a mixture of the epoxy formulation before curing or they can be precoated onto the surface of the filler material. Excellent results are obtained in each case.